The objective of this research is to determine the nature of the critical period for imprinting to an arbitrary stimulus and to a stimulus, the maternal call, to which the subjects are predisposed to respond. That is, to determine (a) whether there is a period during which imprinting is strongest and least subject to reversal, and (b) whether the characteristics of this period differ as a function of the stimuli used. Groups of Peking ducklings 12, 24, 48, and 96 hours of age are trained to a criterion of following of one of two arbitrary stimuli or one of two maternal calls. The arbitrary stimuli are plain, silent, visual models; the maternal calls are of the same or different species as the subjects. At 120 hours of age, all ducklings are given preference tests between the stimulus which they followed and the other of the same type; that is, between the two visual models or between the two maternal calls. These tests will measure strength of imprinting as a function of age. Following testing, the ducklings are trained to a criterion of following of the visual model or maternal call that is not used in original training. Time to the criterion of following in this reversal training is a second measure of strength of imprinting. A second series of preference tests is then given to measure the extent to which reversal training produced a shift in the preference shown after original training.